"Conventional" electromagnetic locks mount with the face of the electromagnet co-planar with that of the door. The electromagnet body mounts on the door frame with an armature plate mounted to the door. When the door closes, the armature plate abuts directly against the face of the electromagnet, and an electromagnetic force secures the door. Within the industry, this is sometimes referred to as a "direct pull" electromagnetic lock.
A second more specialized electromagnetic lock also exists called a "shear lock". With this type of lock, the face of the electromagnet is perpendicular to the plane of the door. When the armature plate is secured to the electromagnet, an attempt to open the door results in a sliding force being applied to the electromagnetic bond. Such a door securing technique has two advantages over conventionally mounted electromagnetic locks: the door can still swing in both directions which is required for double acting or revolving doors, and the lock can be completely concealed in the door and door frame which is more aesthetically pleasing.
Shear locks are intrinsically more complex than conventional electromagnetic locks for several reasons. First, electromagnetic force acting in shear is insufficient to secure a door so it must be aided by some means of mechanical engagement. Second, the armature plate must be allowed to move towards and away from the electromagnet so as to first secure and then to decouple the mechanical engagement means. Third, the shear lock system must generally include a door position detection means and often a timer to ensure that the electromagnet is only energized when the door is positioned accurately in a fully closed position.
To amplify this last point, in a conventional magnetic lock installation an external control switch will release power to the electromagnetic lock for entry or exit. The external switch may be operated momentarily or may have a time delay associated with it. In either case if the switch recloses (restoring power to the electromagnet) prior to the door reclosing, the lock will still operate correctly in that it will automatically relock the door when the door does reclose. This automatic relocking occurs when the armature plate slaps against the electromagnet face.
Prior shear locks, however, cannot be re-energized before the door has settled into its final and fully closed position. As the specification of U.S. Pat. No. 5,141,271 issued to Geringer explains, "Energizing the electromagnet before proper armature alignment can cause improper locking or non-locking of the door." This is because as the armature begins to move under the electromagnet, a portion of the armature will be attracted prematurely to the electromagnet face. This partial coupling of the armature and electromagnet will not engage the mechanical engagement means and the door will be awkwardly in an "in between" state, i.e., in a position that is not open but is not fully closed. It is certainly not properly locked but, to the end user, it feels stuck in a partially open position. The end user may leave the door in such a "partially locked" state in which case the door will not be secure. In such a case the user may feel that the lock has failed and contact his supplier for a replacement.
Another early shear lock is disclosed in U.S. Pat. No. 4,487,439 issued to McFadden. This shear lock sought to deal with the problem of incomplete/improper locking by pre-tilting the angle of the armature plate via the action of a spring as shown in FIG. 6 of the patent. This would, in theory, move the edge of the armature plate away from the electromagnet as the door is closing and thereby avoid that edge being attracted early to the electromagnet body. However, this design did not prove commercially practical largely owing to the lack of positional and movement precision that is inherent in ordinary doors. The slight tilt that could be attained was not sufficient to suppress improper "early" engagement of the armature to the electromagnet. It is believed that the owner of the McFadden patent, Dynametric, Inc., sold its designs to Von Duprin Inc. in the mid 1980's. Von Duprin has released commercial shear locks since that time without the tilting feature. An example of a Von Duprin design without the tilting feature is disclosed in Von Duprin's subsequent patent, U.S. Pat. No. 5,184,855 issued to Waltz. This patent relies upon a door position sensing means to avoid improper locking.
Prior art shear locks, other than the unsuccessful McFadden design, included door position sensing means which, through various control circuits, inhibit the electromagnet from energizing until the door is in its proper closed position. An example is U.S. Pat. No. 4,439,808 issued to Gillham, which discloses "means preferably include a proximity switch to provide an indication when the two relatively movable members are not in the predetermined relative position. This prevents false locking . . . " A number of other prior art patents focus on other novel aspects of shear lock design without addressing the requirement for door position sensing, although the door position sensing feature exists in corresponding commercial product designs.
Door position sensing does not, however, always work satisfactorily for a number of reasons. First, doors are not precision devices. Second, the most common door position sensing means is via a proximity switch consisting of a reed switch and a permanent magnet. This type of position sensing maintains an accuracy of only about plus or minus 1/8 inch (about 0.32 cm). Accordingly, the clear possibility exists that the door will still improperly lock owing to the limited accuracy of typical door position sensing means.
A common approach in prior shear locks is to incorporate a timer into the lock control circuitry. As the door recloses, the door position sensing means detects that the door is nearly in the closed position and activates a timer which is typically set for a few seconds. The timer maintains the lock in its de-energized condition. This brief delay is hoped to be sufficient for the door to settle into its proper closed position. After the delay, the lock is re-energized. This technique can fail if the door fails to find a proper closed position. This risk is greatest with a swing through or double acting door; however, that type of door constitutes a prime use for shear locks. Even on conventional doors, such factors as air pressure differentials and aging door closers commonly prevent doors from closing accurately. Another failure mechanism is if the door is moved by a person just as the control timer is timing out. The door may then become "partially locked" as described earlier. While it may seem that the chance of someone attempting to use the door just as the lock delay is timing out would be remote, electromagnetic locks--either conventional or shear--have long operating lives and may be used hundreds of times each day so even rare functional failures present a significant problem to the end user.
A second limitation of prior shear locks is "position sensitivity". Prior shear locks were designed such that the armature plate is mounted beneath the electromagnet. When the lock is de-energized, gravity plays a crucial role in separating the armature plate from the electromagnet. The present invention can be mounted with the armature beneath the electromagnet or with the electromagnet and armature facing each other on the vertical portion of the door frame and door. This is particularly useful as the end user can mount the lock half way up the vertical door frame at about 3 1/2 feet above the floor. This is the same position where the door knob or door lever handle is mounted. In this position the proximity of the lock to the door knob position gives an impression of the door being tightly locked to someone pulling or pushing on the knob. When the lock is mounted at the top of the door as is the case with prior shear locks, pulling or pushing on the door knob causes the door to flex, giving an impression of low security. Such flexing, when continued over a long period of time, can also permanently bend the door.
Additionally, prior shear locks cannot be used in a specialized application: electric sliding doors with emergency push-out release. Such doors are often found in supermarkets. Ordinarily, such doors slide open to admit customers when they are triggered by a motion sensor or pressure mat. In a fire or other emergency, however, power could be lost and the doors would no longer slide open to permit evacuation. Such doors therefore include an emergency "push out" capability whereby a person needing to escape in a panic situation can push the doors open without the need to apply heavy force. This makes the doors insecure against break in. To overcome that weakness, the doors are generally mechanically locked after hours; however, many owners of such door would prefer electric locking. It is believed that to date no electric lock has been able to provide the desired dual motion of "sliding/push-out" doors.